I have recently had an issue with this, so would like to pass on the lesson I learnt! Buy big first time!
I initially had only RFs and bought a small safe from Screwfix I think it was for £30. Worked perfectly. But then I got in to CFs and needed something bigger as I had a locked military ammo box as well as the small safe and it was all getting a bit messy.
Bought the Buffalo River one, £55 in my local gunshop. Wish I had done this straight away! Can get bolts, silencers, magazines and enough ammo for all 3 calibres to start WW3! Also when we go away the valuables can go in there too.